U.S. patent number 4,702,836 [Application Number 06/884,519] was granted by the patent office on 1987-10-27 for porous fluorine resin membrane and process for preparing the same.
This patent grant is currently assigned to Asahi Kasei Kogyo Kabushiki Kaisha. Invention is credited to Morikazu Miura, Yoshihiko Mutoh.
United States Patent |
4,702,836 |
Mutoh , et al. |
October 27, 1987 |
Porous fluorine resin membrane and process for preparing the
same
Abstract
The invention relates to a porous membrane made of a particular
fluorine resin, having excellent chemical resistance, excellent
thermal resistance, excellent filtration performance and excellent
mechanical properties, and having uniform porous structure
comprising minute pores, and also relates to a process for
preparing the same. In particular, the invention relates to a
porous membrane suited for a microfilter having excellent thermal
resistance and excellent filtration performance, especially a
porous membrane suited for a microfilter for use in purification of
chemicals such as strong acid and strong alkali by utilizing the
excellent chemical resistance, and also a process for preparing the
same.
Inventors: |
Mutoh; Yoshihiko (Fujisawa,
JP), Miura; Morikazu (Yokohama, JP) |
Assignee: |
Asahi Kasei Kogyo Kabushiki
Kaisha (Osaka, JP)
|
Family
ID: |
26414619 |
Appl.
No.: |
06/884,519 |
Filed: |
July 11, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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808491 |
Dec 13, 1985 |
4623670 |
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Foreign Application Priority Data
|
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Dec 27, 1984 [JP] |
|
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59-273920 |
Apr 9, 1985 [JP] |
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60-73471 |
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Current U.S.
Class: |
210/500.23;
210/500.36; 521/134; 521/145; 521/61; 521/64 |
Current CPC
Class: |
B01D
67/003 (20130101); B01D 71/32 (20130101); C08J
9/26 (20130101); B01D 67/0027 (20130101); C08J
2323/08 (20130101); C08J 2327/12 (20130101); C08J
2201/0442 (20130101) |
Current International
Class: |
B01D
71/00 (20060101); B01D 71/32 (20060101); B01D
67/00 (20060101); C08J 9/26 (20060101); C08J
9/00 (20060101); B01D 013/00 () |
Field of
Search: |
;210/500.23,500.36
;521/61,64,134,145 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Foelak; Morton
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch
Parent Case Text
This application is a division of co-pending application Ser. No.
808,491, filed on Dec. 13, 1985, now U.S. Pat. No. 4,623,670.
Claims
What is claimed is:
1. A porous fluorine resin membrane comprising a fluorine resin
selected from the group consisting of an
ethylene-tetrafluoroethylene copolymer, an
ethylene-chlorotrifluoroethylene copolymer and
polychlorotrifluoroethylene, having a mean pore size of from 0.01
to 5.mu. and a porosity of 40 to 90%, and having a three
dimensional network structure such that the symbol N designating
the number of fibers constituted a network is N.gtoreq.2p/D per 1
mm in the thicknesswise direction of the membrane, wherein p
represents a porosity (%) and D represents mean pore side
(.mu.).
2. The porous fluorine resin membrane according to claim 1, wherein
the number N of fibers constituted the network is such that
N.gtoreq.5p/D per 1 mm in the thicknesswise direction of the
membrane.
3. The porous fluorine resin membrane according to claim 1, wherein
said porous membrane comprises a hollow fiber.
Description
BACKGROUND OF THE INVENTION
This invention relates to a porous fluorine resin membrane and a
process for preparing the same.
As porous fluorine type resin membrane having excellent chemical
resistance and thermal resistance, there have been known the
following ones. Japanese Unexamined Patent Publications No.
136354/1975, No. 158465/1979 and No. 147030/1984 are known to
disclose porous membranes made of an ethylene-tetrafluoroethylene
copolymer and processes for preparing the same. The Japanese
Unexamined Patent Publication No. 136354/1975 discloses a process
in which fine powder of an ethylene-tetrafluoroethylene copolymer
is subjected to styrene polymerization by preparing a mixed
solution of a styrene monomer and a slurry to form a membrane, from
which a styrene polymer is eluted to form a porous membrane. The
porous membrane obtained by this process, however, has a pore size
of as large as 10.mu. and, moreover, has very poor permeability,
thereby being unsuitable for a microfilter. The Japanese Unexamined
Patent Publication No. 158465/1979 discloses a process in which a
film of an ethylene-tetrafluoroethylene copolymer is subjected to
charging particle irradiation, followed by etching with an aqueous
solution of sodium hydroxide to form a porous membrane. However,
the porous membrane thus obtained, having no three dimensional
network structure, is not only poor in its performance but also
inferior in the mechanical properties, thereby making it impossible
to obtain a uniform, hollow fibrous porous membrane. Moreover,
there is a problem that the process is not suited for a mass
production since it uses a nuclear reactor. The Japanese Unexamined
Patent Publication No. 147030/1984 discloses a process in which a
film of an ethylene-tetrafluoroethylene copolymer is coated with a
resist to form a resist pattern having holes, and thereafter a
thru-hole corresponding to the resist pattern is formed by sputter
etching treatment to obtain a porous membrane. The porous membrane
obtained by this process, however, which also has no three
dimensional network structure, is also not only poor in its
performance but also inferior in the mechanical properties, thereby
making it difficult to obtain a uniform, hollow fibrous porous
membrane. Moreover, there is a problem in the productivity since
the process requires the sputter etching over a long period of
time.
As a process having solved the above problems, Japanese Unexamined
Patent Publications No. 79011/1980 No. 159128/1981, No. 28139/1982,
No. 93798/1983, No. 179297/1983, etc. are known to disclose a
process in which an ethylene-tetrafluoroethylene copolymer, fine
powder of silica and dioctylphthalate are mixed and melt-molded,
followed by extraction of the fine powder of silica and
dioctylphthalate from the molded product to form a porous membrane.
However, the porous membrane obtained by this process has not
sufficient uniformity in the pore structure, and contains a number
of voids which are extraordinarily large as compared with its mean
pore size. Accordingly, although this porous membrane is a membrane
having a three dimensional network structure, the number of fibers
constituting the network is small in the thicknesswise direction of
the membrane, and the performance to eliminate fine particles is
inferior when it is used as a microfilter. Moreover, this porous
membrane generates pinholes (abnormal coarse communicated holes)
very frequently, and has a problem of inconsistency in the quality
(great variance in performance) of the membrane and inferiority in
the productivity (yield of an article of good quality).
Thus, as mentioned above, none of the conventionally available
porous membranes made of an ethylene-tetrafluoroethylene copolymer
and processes for preparing the same have been satisfactory.
As a porous membrane made of polychlorotrifluoroethylene, there is
a diaphragm membrane for use in electrolysis as disclosed in
Japanese Unexamined Patent Publications No. 34081/1972 and No.
25065/1973. However, these membranes, which are used as diaphragm
membranes for electrolysis, have extremely low permeability, and
are not suited for a porous membrane for use in a microfilter.
Accordingly, none of the conventionally available porous membranes
made of polychlorotrifluoroethylene and processes for preparing the
same have been satisfactory.
Further, nothing has been known conventionally as to membranes
comprising an ethylene-chlorotrifluoroethylene copolymer and
processes for preparing the same.
SUMMARY OF THE INVENTION
This invention relates to a porous membrane having excellent
chemical resistance, thermal resistance and mechanical properties,
having a uniform porous structure comprising minute pores, and
having excellent permeability. An object of this invention is to
provide a porous membrane which makes it possible to carry out
highly precise filtration purification such as thermal conc.
sulfuric acid filtration where severe conditions for the thermal
resistance and chemical resistance are imposed. Another object of
this invention is to provide a process for preparing such a porous
membrane in good productivity.
This invention is principally characterized by a porous fluorine
resin membrane comprising a fluorine resin selected from the group
consisting of an ethylene-tetrafluoroethylene copolymer, an
ethylene-chlorotrifluoroethylene copolymer and
polychlorotrifluoroethylene, having a mean pore size of from 0.01
to 5.mu.0 and a porosity of 40 to 90%, and having a three
dimensional network structure such that the symbol N designating
the number of fibers constituting a network is N.gtoreq.2p/D per 1
mm in the thicknesswise direction of the membrane, wherein p
represents porosity (%) and D represents mean pore size (.mu.). It
is also characterized by a process for preparing a porous fluorine
resin membrane, which comprises mixing 10 to 60% by volume of a
fluorine resin selected from the above-mentioned group, 7 to 42% by
volume of an inorganic fine powder material and 30 to 75% by volume
of a chlorotrifluoroethylene oligomer, followed by melt-molding to
form a molded product, removing by extraction the
chlorotrifluoroethylene oligomer from said molded product, and
further removing by extraction therefrom the inorganic fine powder
material. It is further characterized by a process for preparing a
porous fluorine resin membrane, which comprises mixing 10 to 60% by
volume of a fluorine resin selected from the above-mentioned group,
7 to 42% by volume of an inorganic fine powder material and 30 to
75% by volume of a mixture of a chlorotrifluoroethylene oligomer
and an organic heat-resistant substance having the SP value of 5 to
11 other than the chlorotrifluoroethylene oligomer, followed by
melt-molding to form a molded product, removing by extraction the
chlorotrifluoroethylene oligomer and the organic heat-resistant
substance from said molded product, and further removing by
extraction therefrom the inorganic fine powder material.
DETAILED DESCRIPTION OF THE INVENTION
In this invention, an ethylene-tetrafluoroethylene copolymer, an
ethylene-chlorotrifluoroethylene copolymer and
polychlorotrifluoroethylene which are excellent in the chemical
resistance, thermal resistance and mechanical resistance are
selected as the material for the porous membrane. A mixture of
these resins may also be used.
The ethylene-tetrafluoroethylene copolymer is a copolymer wherein
ethylene and tetrafluoroethylene principally are combined
alternately. Its melting point varies depending on the
compositional proportion of ethylene to tetrafluoroethylene and it
ranges from about 200.degree. C. to about 280.degree. C. One having
a higher melting point is more preferred. If the copolymer is
substantially an ethylene-tetrafluoroethylene copolymer, it is
permissible to use a copolymer thereof with a third component such
as hexafluoropropylene or a copolymer wherein a stabilizing agent
or the like for preventing deterioration of the polymer is
incorporated. Commercially available products include Aflon COP
(trademark, produced by Asahi Garasu K.K.), Nefolon ETFE
(trademark, produced by Daikin Kogyo K.K.), Tefzel (trademark,
produced by Du Pont), Hostaflon (trademark, produced by Hoechst),
etc.
The ethylene-chlorotrifluoroethylene copolymer is a copolymer
wherein ethylene and chlorotrifluoroethylene principally are
combined alternately. Its melting point varies depending on the
compositional proportion of ethylene to chlorotrifluoroethylene and
it ranges from about 200.degree. C. to about 260.degree. C. One
having a higher melting point is more preferred. If the copolymer
is substantially an ethylene-chlorotrifluoroethylene copolymer, it
is permissible to use a copolymer thereof with a third component or
a copolymer wherein a stabilizing agent or the like for preventing
deterioration of the polymer is incorporated. Commercially
available products include Halar (trademark, produces by Allied
Corp.), etc.
The polychlorotrifluoroethylene is a polymer of
chlorotrifluoroethylene. Its melting point ranges from 210.degree.
C. to 220.degree. C. If is is substantially a
polychlorotrifluoroethylene, it is permissible to use a copolymer
with a second component or a polymer wherein a stabilizing agent or
the like for preventing deterioration of the polymer is
incorporated. Commerically available products include Daiflon
(trademark, produced by Daikin Kogyo K.K.), Aclon CTFE (trademark,
produced by Allied Corp.), Kel-F (trademark, produces by 3M),
Voltalef (trademark, produced by Ugine Kuhlmann), etc.
In the porous membrane according to this invention, the mean pore
size ranges preferably from 0.01 to 5.mu., more preferably from
0.05 to 1.mu.. If it is less than 0.01.mu., the permeability
becomes too low, and if it exceeds 5.mu., the performance to
eliminate fine particles becomes inferior, both of which cases are
thus undesirable. The porosity preferably ranges from 40 to 90%. If
it is less than 40%, the permeability becomes too low, and if it
exceeds 90%, the mechanical properties are seriously worsened, both
of which cases are thus undesirable.
The porous membrane according to this invention has a three
dimensional network structure. The three dimensional network
structure herein refers to a porous structure in which the network
structure constituted of resin is observed on the surface and at
every section of the porous membrane, namely the so-called spongy
structure in which pores constituting the porous structure are
continuously communicated with each other in the interior of the
membrane. Such a three dimensional network structure gives
excellence in the strength as a porous membrane, and, when the
porous membrane is used for a filter, it also gives excellence in
the particle-rejection property to eliminate impurity particles
such as dust by the same effect as in the case where screens are
overlapped in a large number.
Further, the fiber constituting the network structure herein refers
to a netlike resin portion surrounding a pore. Although it is
termed as "fiber" for convenience sake, no particular limitation is
given to its shape, and, other than the fibrous structure, the
shape may be of laminated structure, knotted structure or amorphous
structure if the three dimensional network structure can be
formed.
It is required in this invention that the symbol N designating the
number of fibers constituting a network is N.gtoreq.2p/D per 1 mm,
preferably N.gtoreq.5p/D per 1 mm, in the thicknesswise direction
of the membrane, wherein p represents porosity (%) and D represents
mean pore size (.mu.). If N<2p/D, the membrane becomes inferior
undesirably in the particle rejection effect to eliminate impurity
particles such as dust when it is used as a filter.
Membrane thickness of the porous membrane of the invention
preferably ranges from 0.025 to 2.5 mm. If it is less than 0.025,
the mechanical properties become inferior, and if it exceeds 2.5
mm, the permeability becomes inferior, both of which cases are thus
undesirable. The membrane may take the shape of a hollow fiber, a
tube, a flat membrane, etc., but, in view of its use for a
microfilter, it is preferably in the shape of a hollow fiber for
the reason of compactness of a device in which the membrane is
assembled into a module.
Features of the process for preparing the porous membrane according
to this invention will be described below.
In this invention, there may be used a fluorine resin comprising an
ethylene-tetrafluoroethylene copolymer, an
ethylene-chlorotrifluoroethylene copolymer or
polychlorotrifluoroethylene, an inorganic fine powder material, and
a chlorotrifluoroethylene oligomer or a mixture of a
chlorotrifluoroethylene oligomer and a heat-resistant substance
having the SP value of 5 to 11 other than the
chlorotrifluoroethylene oligomer.
The inorganic fine powder material preferably comprises fine
particles having specific surface area of 50 to 500 m.sup.2 /g and
a mean particle size being in the range of from 0.005 to 0.5.mu..
Useful inorganic fine powder material includes silica, calcium
silicate, aluminum silicate, magnesium oxide, alumina, calcium
carbonate, kaolin, clay, diatomaceous earth, etc. Of these, fine
powder of silica is preferred in particular.
In the process according to the invention, it is essential to use
the chlorotrifluoroethylene oligomer, and it has been made possible
for the first time by using it to prepare a porous membrane having
uniform pore structure and excellent particle-rejection property to
remove impurity particles such as dust, with constant quality of
membrane and with good productivity. The chlorotrifluoroethylene
oligomer is preferably a tetramer to twentymer (4 to 20 mers), more
preferably an octamer to pentadecamer (8 to 15 mers), most
preferably a nonamer to dodecamer (9 to 12 mers) of
chlorotrifluoroethylene. If it is a trimer (3 mers) or less, the
thermal resistance becomes poor to cause enormous evaporation at
the time of melt-molding, and the permeability of the porous
membrane becomes smaller undesirably. Also, if it is 21 mers or
more, the mixing workability is worsened and the extractability is
also worsened undesirably. The mer number of the
chlorotrifluoroethylene oligomer used in this invention may refer
to an average mer number when it is a chlorotrifluoroethylene
oligomer constituted of a mixture of chlorotrifluoroethylene
oligomer having various mer numbers.
In this invention, it becomes easy to control the pore size of the
porous membrane by using a mixture of chlorotrifluoroethylene
oligomer and a heat-resistant organic substance having the
solubility parameter (hereinafter "SP value") of 5 to 11 other than
the chlorotrifluoroethylene oligomer. Namely, it becomes possible
to readily control the pore size to a desired pore size by making
selection of the heat-resistant organic substance and/or regulation
of the mixing proportion of the chlorotrifluoroethylene oligomer to
the heat-resistant organic substance. In particular, in a process
for preparing porous membranes of polychlorotrifluoroethylene, it
is preferable to use the mixture of chlorotrifluoroethylene
oligomer and the other heat-resistant organic substance since the
pore size may become small to lower the permeability when the
chlorotrifluoroethylene oligomer is used alone.
Mixing ratio of the chlorotrifluoroethylene oligomer to the other
heat-resistant organic substance may vary depending on the kind of
the heat-resistant organic substance, but, in general, it is
preferably 10 volumes or less, preferably 4 volumes or less, based
on the chlorotrifluoroethylene oligomer. If it exceeds 10 volumes,
the pore size of a membrane obtained tends to become large, and the
membrane is liable to have a non-uniform structure and also tends
to frequently generate pinholes (abnormal coarse communicated
holes). When the heat-resistant organic substance is silicone oil,
the above mixing ratio is preferably 2 volumes or less.
If a mixture of a chlorotrifluoroethylene oligomer and a
heat-resistant organic substance having the SP value exceeding 11
is used, the pore size of the membrane obtained becomes undesirably
too large for the membrane to have uniform pore structure since the
compatibility of the chlorotrifluoroethylene oligomer with the
heat-resistant organic substance having the SP value exceeding 11
is poor. (A heat-resistant organic substance having the SP value of
less than 5 has not been available in the art.)
The heat-resistant organic substance in this invention is an
organic substance which has the thermal resistance such that the
boiling point at 1 atm. is at least 200.degree. C. or more,
preferably 250.degree. C. or more, and in the form of a liquid at
the time of melt-molding of the porous membrane of the invention.
The heat-resistant organic substance having the SP value of 5 to 11
may include silicon oil, a perfluoropolyether oligomer, a phthalic
acid ester, a trimellitic acid ester, sebacic acid ester, an adipic
acid ester, an azelaic acid ester, a phosphoric acid ester, etc. Of
these, preferred are silicone oil, a perfluoropolyether oligomer
and a trimellitic acid ester. In particular, silicone oil is more
preferred in view of its stability at the time of melt-molding,
cost, etc. Silicone oil is a heat-resistant organic substance
having a siloxane structure and includes dimethyl silicone,
methylphenyl silicone, diphenyl silicone, etc.
The mixing ratio of a chlorotrifluoroethylene oligomer with a
heat-resistant organic substance having a SP value of 5 to 11
except for chlorotriethylene oligomer may differ depending on the
kind of the heat-resistant organic substance, but may generally be
preferred to be 10 volumes of a heat-resistant organic substance
per one volume of the chlorotrifluoroethylene oligomer, more
preferably 4 volumes or less. If it exceeds 10 volumes, the
membrane obtained tends to have a greater pore size and also a
nonuniform pore structure, and pinholes (abnormal coarse connected
pores) also tend to occur frequently. When the heat-resistant
organic substance is silicone oil, it should preferably be employed
in 2 volumes or less.
In preparing the porous membrane of this invention, prepared first
is a mixture of a fluorine resin comprising an
ethylene-tetrafluoroethylene copolymer, an
ethylene-chlorotrifluoroethylene copolymer or
polychlorotrifluoroethylene, an inorganic fine powder material and
a chlorotrifluoroethylene oligomer or a mixture thereof with a
heat-resistant organic substance having the SP value of 5 to 11
other than the chlorotrifluoroethylene oligomer. The mixing
proportion is 10 to 60% by volume, preferably 15 to 40% by volume
of the fluorine resin; 7 to 42% by volume, preferably 10 to 20% by
volume of the inorganic fine powder material; and 30 to 75% by
volume, preferably 50 to 70% by volume of the
chlorotrifluoroethylene oligomer or the mixture thereof with the
heat-resistant organic substance. If the fluorine resin is present
in less than 10% by volume, the resin amount is so small that the
strength may become small and also the moldability may become poor.
If it is in excess of 60% by volume, a porous membrane having a
large porosity is not obtained undesirably. If the inorganic fine
powder material is present in less than 7% by volume, the molding
becomes difficult to carry out, and if it is in excess of 42% by
volume, the fluidity at the melting step becomes poor to obtain
only a molded product which is too brittle to be put into a
practical use. If the chlorotrifluoroethylene oligomer or the
mixture thereof with the heat-resistant organic substance is
present in less than 30% by volume, the porosity of a porous
membrane obtained becomes lower than 40% with the result that no
porous membrane having excellent permeability can be obtained, and
if it is in excess of 75% by volume, the molding becomes so
difficult to carry out that no porous membrane having high
mechanical strength can be obtained.
Mixing of the above respective components is carried out by means
of a mixing machine such as a Henschel mixer, a V-blender and a
ribbon blender. As for the order of mixing, it is preferable to
first mix the inorganic fine powder material and the
chlorotrifluoroethylene oligomer or the mixture thereof with the
heat-resistant organic substance and then incorporating the
fluorine resin for mixing, rather than mixing the respective
components simultaneously. This mixture is preferably further
kneaded by means of a melting and kneading device such as an
extruder. The kneaded product thus obtained is crushed with a
crusher if desired and then melt-molded into a flat membrane or a
hollow fiber membrane by means of an extruder. It is also possible
to directly mold the mixture by means of a device having both
kneading and extruding functions, such as a kneader-extruder.
Next, the chlorotrifluoroethylene oligomer or the mixture thereof
with the heat-resistant organic substance used in combination is
extracted from the membranous molded product with use of a solvent.
When the mixture of the chlorotrifluoroethylene oligomer with the
heat-resistant organic substance substance is used, both of the two
components are preferably extracted simultaneously, but
alternatively may be extracted separately for each component. The
extraction may be carried out according to an orginary method for
extraction, employed for a membranous product, such as a batch
method and a countercurrent multistage method. The solvent used for
the extraction may preferably include a halogenated hydrocarbon
such as 1,1,1-trichloroethane and tetrachloroethylene.
After finishing the extraction of the trifluoroethylene oligomer or
the mixture thereof with the heat-resistant organic substance, a
half-extracted porous membrane is further subjected to extraction
of the inorganic fine powder material with use of a solvent for
inorganic fine powder material. Extraction is carried out according
to an ordinary method for extraction such as a batch method and a
countercurrent multistage method, and is finished in several
seconds to several tens of hours. The solvent used for the
inorganic fine powder material includes an acid such as
hydrochloric acid, sulfuric acid and hydrofluoric acid for
extraction of calcium carbonate, magnesium carbonate, magnesium
oxide, calcium silicate, magnesium carbonate, etc. and an aqueous
alkali solution such as sodium hydroxide and potassium hydroxide
for extraction of silica. Other solvents may be used without any
particular limitation if they do not substantially dissolve the
fluorine resin, but dissolve the inorganic fine powder
material.
When it is desired to obtain a porous membrane having higher
thermal resistance, it is effective to carry out an annealing
treatment after extraction of the chlorotrifluoroethylene oligomer,
etc. and in a state where the inorganic fine power material is
present in the membrane, and then remove by extraction the
inorganic fine powder material.
In general, when a porous membrane is exposed to high temperature
in order to be assembled into a module or when a filtration under a
high temperature condition is carried out, it often occurs that the
pore size of the porous membrane is varied and the permeability is
lowered.
The present inventors have considered that the lowering of
performance of a porous membrane at a high temperature is
principally caused by a mechanism that, when a porous membrane is
processed for molding, a "distortion" is generated in the interior
of the resin constituting the porous membrane and a cancellation of
the "distortion" occurs at heating, and they have made intensive
studies on a process by which the "distortion" is suppressed to a
minimum extent to prepare a porous membrane affording less lowering
of performance at a high temperature. In general, an annealing is
carried out in order to cancel such "distortion". When the
annealing is carried out in a conventional manner on a porous
membrane constituted of a resin only, the properties are greatly
changed, and yet non-uniformly changed at every portion of the
membrane. As a result of change in the shape of the membrane, the
resultant membrane is of non-uniformity and tends to be one which
can not achieve reproducibility. In order not to cause such a
non-uniform change in the physical properties, the membrane may be
restrained by some means so as not to cause the change in the shape
of the membrane, but in general it is difficult to restrain a
membrane externally. Even if it is a flat membrane, it is difficult
to restrain the membrane in the thicknesswise direction, although
possible in the longitudinal and lateral directions. In a hollow
fibrous porous membrane, it is much more difficult to restrain the
membrane in the direction other than the longitudinal direction,
and it is also difficult to obtain a uniform membrane by applying
an annealing.
The present inventors have found that a porous membrane in a state
being filled with an inorganic fine powder material may be
subjected to an annealing in a hot-air oven or the like, whereby
the inorganic fine powder material itself restrains internally the
shape of a porous membrane, and as a result a uniform membrane can
be obtained with good reproducibility. Temperature for the
annealing may be a temperature higher than the glass transition
point of the resin, but it is preferably in the range of from the
melting point of the resin to -100.degree. C. from the viewpoint of
the productivity or the time required for the annealing. It is more
effective and desirable to conduct the treatment at a temperature
higher than the actual temperature to be expected (including the
heated conditions at the step of assembling a module). Time for
annealing treatment, although being correlated with the treatment
temperature, may be usually in the range of from several second to
several days.
When the improvement in the thermal resistance of a porous membrane
by the annealing is insufficient, another annealing may be carried
out again after the first annealing was applied and the inorganic
fine powder material was removed by extraction, whereby the thermal
resistance of a porous membrane may be further improved.
In this invention, in order to enlarge the pore size, increse the
porosity or improve the mechanical properties of a porous membrane,
the porous membrane from which one or both of the
chlorotrifluoroethylene oligomer or the mixture thereof with the
heat-resistance organic substance used in combination, and the
inorganic fine powder material has been extracted may be drawn
monoaxially or biaxially.
The properties indicated in this invention are determined in
accordance with the following measurement methods.
Mean Pore Size (.mu.)
Pore sizes are measured by an electronmicroscopic observation on
the surfaces and the section of a specimen, and then averaged
(number average).
Porosity (%)
Determined by the following equation: ##EQU1## wherein the void
volume is obtained by subtracting the weight of a porous material
only, from the weight of a porous material filled with water in the
pores of the porous material.
Three Dimensional Network Structure
Visually judged by a microscopic observation using a scanning type
electron microscope.
The Number N of Fibers Constituting the Network (number/mm):
A membrane is sectionally observed by an electron microscope to
count the number of fibers in the thicknesswise direction of the
membrane and calculated as the number of fibers per 1 mm length of
the membrane. When there is irregularity in the porous structure,
the number is measured at several portions and then averaged.
SP Value (solubility parameter):
Calculated by the following equation (Small's equation): ##EQU2##
wherein D: specific gravity, G: molar attraction constant* and M:
molecular weight.
Indices relating to the practical performance of a membrane may
include the following.
Frequency of Generation of Pinholes (number/m):
The number of pores extraordinarily coarse is evaluated. This is an
evaluation item for the uniformity in the porous structure. A
continuously hollow fibrous porous membrane of 150 m is immersed in
ethyl alcohol and a pressure which is 0.5 kg/cm.sup.2 lower than
the bubble point pressure (measured according to the ASTM F316-80)
of the porous membrane is applied on the interior of one side of
the hollow fiber, and the number of bubbles generated under such
situation is checked to calculate the frequency according to the
following equation: ##EQU3##
Particle Rejection (%):
This is a factor relating to the performance to eliminate fine
particles which are rejected from passing, i.e. eliminated, by a
porous membrane when filtration is performed. An aqueous solution
in which Uniform Latex Particles available from Dow Chemical Co.
are diluted to have the solid concentration of 0.01% by weight is
filtered through a porous membrane, and the concentration of Latex
Particles in the solution having passed the membrane is evaluated
to determine the proportion of elimination of the Latex
Particles.
Water Permeation Quantity (lit/m.sup.2, hr atm. 25.degree. C.):
This is a factor relating to the quantity of water passing through
a porous membrane when filtration is performed, and is measured at
25.degree. C. and pressure difference of 1 kg/cm.sup.2.
The porous fluorine resin type membrane according to this invention
assumes uniform porous structure, and has excellent permeability,
chemical resistance, thermal resistance and durability, capable of
achieving highly precise filtration purification. Also, the
preparation process according to this invention can produce the
porous membrane with high efficiency.
The present invention will be further illustrated by but is not
intended to be limited to the following examples wherein parts and
percentages are by volume.
EXAMPLE 1
11.1 % by volume of fine poweder of silica (Aerosil R-972,
trademark, produced by Japan Aerosil Co.; specific surface area:
120 m.sup.2 /g; mean particle size: 16 .mu.m) and 62.2% by volume
of a chlorotrifluoroethylene oligomer (Daifloil #20, trademark,
produced by Daikin Kogyo K.K.,; about 8 mers) were mixed by use of
a Henschel mixer, to which further added was 26.7% by volume of an
ethylene-tetrafluoroethylene copolymer (Aflon COP Z-8820,
trademark, produced by Asahi Garasu K.K.), and mixing was carried
out again by use of a Henschel mixer.
The mixture obtained was kneaded in a twin extruder of 30 mm .phi.
at 260.degree. C. and formed into pellets. The pellets were molded
at 260.degree. C. into hollow fibers by means of a hollow fiber
preparation apparatus comprising a twin extruder of 30 mm .phi.
equipped with a hollow spinneret. The molded hollow fiber was
immersed in 1,1,1-trichloroethane at 50.degree. C. for 1 hour to
extract the chlorotrifluoroethylene oligomer, followed by
drying.
Subsequently, the product obtained as above was immersed in a 40%
aqueous solution of sodium hydroxide at 70.degree. C. for 1 hour to
extract the fine powder of silica, followed by water-washing and
drying.
The porous membrane thus obtained, made of an
ethylene-tetrafluoroethylene copolymer, had a three dimensional
network structure. Performances thereof are shown in Table 1.
EXAMPLES 2 TO 4
A porous membrane of an ethylene-tetrafluoroethylene copolymer was
obtained in the same manner as in Example 1 except that the
chlorotrifluoroethylene oligomer was replaced by a mixture
comprising dimethyl silicone (Shin-etsu Silicone KF 96, trademark,
produced by Shin-etsu Kagaku Kogyo K.K., SP value: 6.3) in the
volume shown below based on 1 part by volume of a
chlorotrifluoroethylene oligomer (Daifloil #20, trademark, produced
by Daikin Kogyo K.K.; about 8 mers).
______________________________________ (Volume of dimethyl
silicone) ______________________________________ Example 2 0.17
part by volume Example 3 0.20 part by volume Example 4 0.25 part by
volume ______________________________________
The porous membrane obtained each had a three dimensional
structure. Performances thereof are shown in Table 1.
EXAMPLE 5
13.3% by volume of fine powder of silica (Aerosil R-972, trademark,
produced by Japan Aerosil Co.) and 60.0% by volume of a
chlorotrifluoroethylene oligomer (Daifloil #100, trademark,
produced by Daikin Kogyo K.K.; about 11 mers) were mixed by use of
a Henschel mixer, to which was added 26.7% by volume of an
ethylene-tetrafluoroethylene copolymer (Aflon COP Z-8820,
trademark, produced by Asahi Garasu K.K), and mixing was carried
out again by use of a Henschel mixer.
Subsequently, a porous membrane of a ethylene-tetrafluoroethylene
copolymer was obtained in the same manner as in Example 1.
The porous membrane obtained had a three dimensional network
structure with the performances as shown in Table 1.
EXAMPLE 6
In Example 5, annealing treatment was applied at 200.degree. C. for
1 hour in a hot-air circulation type heating chamber after the
chlorotrifluoroethylene oligomer was extracted and the resultant
material was dried. Thereafter, the material thus treated was
immersed in a 40% aqueous solution of sodium hydroxide at
70.degree. C. for 1 hour to extract the fine powder of silica,
followed by water-washing and drying.
The porous membrane thus obtained, made of an
ethylene-tetrafluoroethylene copolymer, had a three dimensional
network structure. The performances thereof are shown in Table
1.
As a standard evaluation of the thermal resistance to examine the
usability of a membrane at high temperature, the membrane obtained
here was allowed to stand for 4 hours in an atmosphere of
180.degree. C. to evaluate its physical properties. The rate of
change relative to the original properties was as small as follows:
water permeation quantity, a decrease of 7%; porosity a decrease of
3%; mean pore size, a decrease of 0%.
For comparison, the membrane obtained in Example 5 having been
applied with no annealing was also allowed to stand for 4 hours in
an atmosphere of 180.degree. C. to evaluate its physical
properties. As a result, the rate of change relative to the
original properties was as large as follows: water permeation
quantity, a decrease of 47%; porosity, a decrease of 13%; mean pore
size, a decrease of 10%.
It is seen from the comparison between Examples 5 and 6 and also
from the foregoing facts, that the thermal resistance of a membrane
can be improved without serious changes in the properties of the
membrane by making the annealing treatment on a porous membrane
which is in a state of being filled with the inorganic fine powder
material.
EXAMPLE 7
14.4% by volume of fine powder silica (Aerosil R-972, trademark,
produced by Japan Aerosil Co.) and 58.9% by volume of a
chlorotrifluoroethylene oligomer (Daifloil #100, trademark,
produced by Daikin Kogyo K.K.) were mixed by use of a Henschel
mixer, to which was added 26.7% by volume of an
ethylene-tetrafluoroethylene copolymer (Neoflon ETFE EP-540,
trademark, produced by Daikin Kogyo K.K.), and mixing was carried
out again by use of a Henschel mixer.
The mixture obtained was kneaded in a twin extruder of 30 mm .phi.
at 260.degree. C. and formed into pellets. The pellets were molded
at 250.degree. C. into a hollow fiber by means of a hollow fiber
preparation apparatus comprising a twin extruder of 30 mm .phi.
equipped with a hollow spinneret. The molded hollow fiber was
immersed in 1,1,1-trichloroethane at 50.degree. C. for 1 hour to
extract the chlorotrifluoroethylene oligomer, followed by
drying.
Thereafter, an annealing was applied at 200.degree. C. for 1 hour
in a hot-air circulation type heating chamber. Subsequently, the
material thus treated was immersed in a 40% aqueous solution of
sodium hydroxide at 70.degree. C. for 1 hour to extract the fine
powder of silica, followed by water-washing and drying.
The porous membrane obtained, made of an
ethylene-tetrafluoroethylene copolymer, had a three dimensional
network structure. The performances thereof are shown in Table
1.
EXAMPLE 8
The porous membrane of an ethylene-tetrafluoroethylene copolymer
obtained in Example 7 was again applied with an annealing at
200.degree. C. for 2 hours.
The porous membrane obtained had a three dimensional network
structure with the performances as shown in Table 1.
This membrane was allowed to stand for 2 hours at 200.degree. C. in
a hot-air circulation type heating chamber to evaluate its physical
properties. As a result, the rate of change relative to the
original properties was found to be as small as follows: water
permeation quantity, a decrease of 4%; porosity, a decrease of 0%;
mean pore size, a decrease of 0%.
For comparison, the membrane obtained in Example 7 having been
applied with no re-annealing was similarly allowed to stand for 2
hours in an atmosphere of 200.degree. C. to evaluate its physical
properties. As a result, the rate of change relative to the
original properties was found to be as large as follows: water
permeation quantity, a decrease of 24%; porosity, a decrease of 4%;
mean pore size, a decrease of 5%.
It is seen from the foregoing that the re-annealing of a porous
membrane is effective for improvement in the thermal resistance of
the membrane.
EXAMPLES 9 TO 11
Porous membranes of an ethylene-tetrafluoroethylene copolymer are
obtained in the same manner as in Example 1 except that the fine
powder of silica, (Aerosil R-972, trademark, produced by Japan
Aerosil Co.), chlorotrifluoroethylene oligomer (Daifloil 100,
trademark, produced by Daikin Kogyo K.K.) and
ethylene-tetrafluoroethylene copolymer (Neoflon ETFE EP-540,
trademark, produced by Daikin Kogyo K.K.) are used in the following
composition.
______________________________________ Composition (% by volume)
Ethylene- tetrafluoro- Fine powder Chlorotrifluoro- ethylene of
silica ethylene oligomer copolymer
______________________________________ Example 9 7 35 58 Example 10
13 72 15 Example 11 21 63 16
______________________________________
The porous membrane obtained each have a three dimensional network
structure with the performances as shown in Table 1.
COMPARATIVE EXAMPLE 1
As a process for preparing a porous membrane of an
ethylene-tetrafluoroethylene copolymer, tested was a process
already known as disclosed in Japanese Unexamined Patent
Publication No. 79011/1980 (U.S. Pat. No. 4,229,297), namely, a
process in which dioctylphthalate is used in place of the
chlorotrifluoroethylene oligomer in this invention.
13.3% by volume of fine powder of silica (Aerosil 200, trademark,
produced by Japan Aerosil Co.; specific surface area: 200 m.sup.2
/g; mean particle size: 16 m.mu.) and 60.0% by volume of
dioctylphthalate was mixed by use of a Henschel mixer, to which was
added 26.7% by volume of the ethylene-tetrafluoroethylene copolymer
(Aflon COP Z-8820, trademark, product by Asahi Garasu K.K), and
mixing was carried out again by use of a Henschel mixer.
The mixer obtained was kneaded in a twin extruder of 30 mm .phi. at
300.degree. C. and formed into pellets. The pellets were molded at
290.degree. C. into a hollow fiber by means of a hollow fiber
preparation apparatus comprising a twin extruder of 30 mm .phi.
equipped with a hollow spinneret.
The molded hollow fiber was immersed in 1,1,1-trichloroethane at
50.degree. C. for 1 hour to extract dioctylphthalate, followed by
drying.
Subsequently, the product treated as above was immersed in a 40%
aqueous solution of sodium hydroxide at 70.degree. C. for 1 hour to
extract the fine powder of silica, followed by water-washing and
drying.
The performances of the porous membrane thus obtained are shown in
Table 1.
The porous membrane obtained here was a nonuniform membrane in
which a large number of large voids was present. As a result of
measurement of 0.085.mu. fine particle rejection, it was as small
as 95%, which was inferior to the membrane of the invention in
Example 4 in which the rejection was 100%. Further, the preparation
of the porous membrane was repeated five times under the same
conditions, with the results that the porous membrane obtained had
mean pore size, of 0.3 to 0.8.mu., water permeation quantity of 650
to 3000 lit/m.sup.2.hr.atm. 25.degree. C. to show that the membrane
performances were seriously changed and the quality of membrane was
not constant.
TABLE 1
__________________________________________________________________________
Hollow fiber Mean Mean Water permeation Pinhole diameter (mm)
membrane pore Number of .alpha. in quantity generation
diameterouter diameterinner (.mu.)thickness (.mu.)size D
(%)Porosity P (number/mm)fibers N ##STR1## atm. 25.degree.
C.)(lit/m.s up.2 .multidot. hr. (number/m)frequen cy
__________________________________________________________________________
Example 1 1.04 0.52 260 0.05 60 7000 5.8 150 0 Example 2 1.00 0.50
250 0.21 65 2100 6.8 1100 0 Example 3 1.02 0.52 250 0.42 67 1200
7.5 1500 0 Example 4 1.00 0.50 250 0.65 67 790 7.7 2400 0 Example 5
1.04 0.52 260 0.22 67 2100 6.9 1600 0 Example 6 1.00 0.48 260 0.20
68 2100 6.2 1700 0 Example 7 1.24 0.77 235 0.21 67 2200 6.9 1700 0
Example 8 1.17 0.73 220 0.20 65 2200 6.8 1300 0 Example 9 1.02 0.52
250 0.29 40 990 7.2 550 0 Example 10 1.00 0.50 250 0.42 81 1400 7.4
2000 0 Example 11 1.02 0.52 250 0.19 82 2600 6.0 1900 0 Comparative
1.00 0.50 250 0.61 68 200 1.8 2300 0.4 Example 1
__________________________________________________________________________
EXAMPLES 12 TO 16
14.4% by volume of fine powder of silica (Aerosil R-972, trademark,
produced by Japan Aerosil Co.) and the compound shown below were
mixed by use of a Henschel mixer, to which was added 26.7% by
volume of an ethylene-chlorotrifluoroethylene copolymer (Halar 920,
trademark, produced by Allied Corp.), and mixing was carried out
again by use of a Henschel mixer.
EXAMPLE 12
58.9% by volume of a chlorotrifluoroethylene oligomer (Daifloil
#20, trademark, produced by Daikin Kogyo K.K.; about 8 mers)
EXAMPLE 13
58.9% by volume of a chlorotrifluoroethylene oligomer (Daifloil
#50, trademark, produced by Daikin Kogyo K.K.; about 9 mers)
EXAMPLE 14
58.9% by volume of a chlorotrifluoroethylene oligomer (Daifloil
#100, trademark, produced by Daikin Kogyo K.K., about 11 mers)
EXAMPLE 15
A mixture of 44.2% by volume of a chlorotrifluoroethylene oligomer
(Daifloil #20, trademark, produced by Daikin Kogyo K.K.) and 14.7%
by volume of dimethyl silicone (Shin-etsu Silicone KF 96,
trademark, produced by Shin-etsu Kagaku Kogyo K.K.)
EXAMPLE 16
A mixture of 29.5% by volume of a chlorotrifluoroethylene oligomer
(Daifloil #20, trademark, produced by Daikin Kogyo K.K.) and 29.4%
by volume of dimethyl silicone (Shin-etsu Silicone KF 96,
trademark, produced by Shin-etsu Kagaku Kogyo K.K.)
The mixture obtained was kneaded in a twin extruder of 30 mm .phi.
at 250.degree. C. and formed into pellets. The pellets were molded
at 230.degree. C. into a hollow fiber by means of a hollow fiber
preparation apparatus comprising a twin extruder of 30 mm .phi.
equipped with a hollow spinneret. The molded hollow fiber was
immersed in 1,1,1-trichloroethane at 50.degree. C. for 1 hour to
extract the chlorotrifluoroethylene oligomer and the dimethyl
silicone, followed by drying. Subsequently, the product thus
treated was immersed in a 40% aqueous solution of sodium hydroxide
at 70.degree. C. for 1 hour to extract the fine powder silica,
followed by water-washing and drying.
The porous membranes obtained, made of an
ethylene-chlorotrifluoroethylene copolymer, each had a three
dimensional network structure with the performances as shown in
Table 2.
TABLE 2
__________________________________________________________________________
Hollow fiber Mean Mean Water permeation Pinhole diameter (mm)
membrane pore Number of .alpha. in quantity generation
diameterouter diameterinner (.mu.)thickness (.mu.)size D
(%)Porosity P (number/mm)fibers N ##STR2## atm. 25.degree.
C.)(lit/m.s up.2 .multidot. hr. (number/m)frequen cy
__________________________________________________________________________
Example 12 1.25 0.66 290 0.08 56 4200 6.0 220 0 Example 13 1.24
0.68 280 0.12 61 3200 6.3 420 0 Example 14 1.26 0.66 300 0.18 65
2300 6.5 920 0 Example 15 1.28 0.62 330 0.32 59 1300 6.8 870 0
Example 16 1.28 0.58 350 2.1 58 200 7.2 28000 --
__________________________________________________________________________
EXAMPLES 17 TO 20
13.3% by volume of fine powder of silica (Aerosil R-972, trademark,
produced by Japan Aerosil Co.) and the compound shown below were
mixed by use of a Henschel mixer, to which added was 26.7% by
volume of polychlorotrifluoroethylene (Daiflon M-300, trademark,
produced by Daikin Kogyo K.K.), and mixing was carried out again by
use of a Henschel mixer.
EXAMPLE 17
60.0% by volume of a chlorotrifluoroethylene oligomer (Daifloil
#20, trademark, produced by Daikin Kogyo K.K.)
EXAMPLE 18
A mixture of 30.0% by volume of a chlorotrifluoroethylene oligomer
(Daifloil #20, trademark, produced by Daikin Kogyo K.K.) and 30.0%
by volume of dimethyl silicone (Shin-etsu Silicone KF 96,
trademark, produced by Shin-etsu Kagaku Kogyo K.K.)
EXAMPLE 19
A mixture of 30.0% by volume of a chlorotrifluoroethylene oligomer
(Daifloil #20, trademark, produced by Daikin Kogyo K.K.) and 30.0%
by volume of tri-(2-ethylhexyl)trimellitate (SP value: 9.0)
EXAMPLE 20
A mixture of 15.0% by volume of a chlorotrifluoroethylene oligomer
(Daifloil #20, trademark, produced by Daikin Kogyo K.K.) and 45.0%
by volume of tri-(2-ethylhexyl)trimellitate
The mixture obtained was kneaded in a twin extruder of 30 mm .phi.
at 250.degree. C. and formed into pellets. The pellets were molded
at 250.degree. C. into a hollow fiber by means of a hollow fiber
preparation apparatus comprising a twin extruder of 30 mm
.phi.equipped with a hollow spinneret. The molded hollow fiber was
immersed in 1,1,1-trichloroethane of 50.degree. C. for 1 hour to
extract the chlorotrifluoroethylene oligomer, dimethyl silicone and
tri-(2-ethylhexyl)trimellitate, followed by drying. Subsequently,
the product thus treated was immersed in a 40% aqueous solution of
sodium hydroxide at 70.degree. C. for 1 hour to extract the fine
powder of silica, followed by water-washing and drying.
The porous membranes obtained, made of polychlorotrifluoroethylene,
each had a three dimensional network structure. The performances
thereof are shown in Table 3.
EXAMPLE 21
11.1% by volume of fine powder of silica (Aerosil R-972, trademark,
produced by Japan Aerosil Co.), 46.7% by volume of a
chlorotrifluoroethylene oligomer (Daifloil #20, trademark, produced
by Daikin Kogyo K.K.) and 15.6% by volume of dimethyl silicone
(Shin-etsu Silicone KF 96, trademark, produced by Shin-etsu Kagaku
Kogyo K.K.) were mixed by use of a Henschel, mixture to which was
added 26.6% by volume of polychlorotrifluoroethylene (Daifloil
M-300, trademark, produced by Daikin Kogyo K.K.), and mixing was
carried out again by use of a Henschel mixer.
Subsequent procedures were followed in the same manner as in
Example 18 to obtain a porous membrane of
polychlorotrifluoroethylene having the three dimensional network
structure. The performances thereof are shown in Table 3.
EXAMPLE 22
In Example 21, an annealing was applied at 200.degree. C. for 1
hour in a hot-air circulation type heating chamber after the
chlorotrifluoroethylene oligomer and dimethyl silicone were
extracted, and the resultant material was dried. Thereafter, the
material thus treated was immersed in a 40% aqueous solution of
sodium hydroxide at 70.degree. C. for 1 hour to extract the fine
powder of silica, followed by water-washing and drying.
The porous membrane obtained, made of polychlorotrifluoroethylene,
had a three dimensional network structure with the performances as
shown in Table 3.
The membrane obtained here was allowed to stand for 1 hour at
180.degree. C. in a hot-air circulation type heating chamber to
evaluate its physical properties. As a result, the rate of change
relative to the original properties was found to be as small as
follows: water permeation quantity, a decrease of 21%; porosity, a
decrease of 9%; mean pore size, a decrease of 9%.
For comparison, the membrane obtained in Example 21 having been
applied with no annealing was also allowed to stand for 1 hour in
an atmosphere of 180.degree. C. to evaluate its physical
properties. As a result, the rate of change relative to the
original properties was as large as follows: water permeation
quantity, a decrease of 75%; porosity, a decrease of 38%; mean pore
size, decrease of 17%.
EXAMPLES 23 TO 25
Porous membranes of polychlorotrifluoroethylene are obtained in the
same manner as in Example 18, except that the fine powder of silica
(Aerosil R-972, trademark, produced by Japan Aerosil Co.),
chlorotrifluoroethylene oligomer (Daifloil #20, trademark, produced
by Daikin Kogyo K.K.), dimethyl silicone (Shin-etsu Silicone KF96,
trademark, produced by Shin-etsu Kagaku Kogyo K.K.) and
polychlorotrifluoroethylene (Daiflon M-300, trademark, produced by
Daikin Kogyo K.K.) are used to have the following composition.
______________________________________ Composition (% by volume)
Fine Chloro- powder trifluoro- Polychloro- of ethylene Dimethyl
trifluoro- silica oligomer silicone ethylene
______________________________________ Example 23 7 18 17 58
Example 24 13 36 36 15 Example 25 21 32 31 16
______________________________________
The porous membrane obtained, made of polychlorotrifluoroethylene,
had a three dimensional network structure with the performances as
shown in Table 3.
TABLE 3
__________________________________________________________________________
Hollow fiber Mean Mean Water permeation Pinhole diameter (mm)
membrane pore Number of .alpha. in quantity generation
diameterouter diameterinner (.mu.)thickness (.mu.)size D
(%)Porosity P (number/mm)fibers N ##STR3## atm. 25.degree.
C.)(lit/m.s up.2 .multidot. hr. (number/m)frequen cy
__________________________________________________________________________
Example 17 2.01 1.02 500 0.01 45 25000 5.5 15 -- Example 18 2.42
1.50 460 0.25 73 2000 7.0 1200 0 Example 19 2.50 1.51 500 0.22 75
2300 6.8 950 0 Example 20 2.51 1.52 500 0.45 75 1200 7.4 2500 0
Example 21 1.02 0.52 250 0.18 52 1900 6.5 570 0 Example 22 1.00
0.48 260 0.22 55 1700 6.7 1090 0 Example 23 2.41 1.49 460 0.31 41
950 7.2 620 0 Example 24 2.40 1.50 450 0.43 82 1400 7.3 2600 0
Example 25 2.39 1.49 450 0.22 83 2600 6.8 1200 0
__________________________________________________________________________
* * * * *